The present invention relates to dynamoelectric machines and, more particularly, to dynamoelectric machines wherein one of the elements is formed by stacking a plurality of lamellae of a magnetic metal.
Although the present invention may be employed to form a stationary or rotating member of a dynamoelectric machine, for concreteness of description, the following description is directed toward the assembly of an annular stator of a large generator. The problems associated with the assembly of corresponding elements of a motor are so similar that one skilled in the art with the present disclosure for reference would be fully enabled to apply the present invention to such apparatus.
Large generators commonly employ a solid forged rotor excited by a source of direct current to produce a magnetic field. The rotor is rotated by torque applied thereto, whereby the magnetic field is also rotated. The rotor rotates within an annulus of magnetic metal containing a plurality of conductor bars set into slots. The conductor bars are interconnected by end turns to form coils. As the magnetic field of the rotor sweeps past the stationary conductor bars, an alternating current is induced in the conductor bars.
If measures were not taken to prevent it, the resulting rapid reversals of magnetic flux in the magnetic metal forming the stator would induce large eddy currents therein, leading to substantial resistive losses. In order to reduce such eddy-current losses to a minimum, a stator is conventionally built up by stacking a plurality of thin (0.014 inch) lamellae of a high-resistance magnetic material such as, for example, silica steel.
Such stacking is conventionally aided by one or more dovetail-shaped key bars affixed inside a stator frame for engaging dovetail-shaped slots in the individual lamellae. During stacking of the stator, the dovetail slots of the lamellae are fitted onto the dovetail-shaped key bars. As the stator is being built up, an axial compressive force is applied to the ends of the stack and maintained for a substantial time to compress the lamellae into a rigid assembly.
Instead of dovetail-shaped engaging elements, it is within contemplation to employ one or more tabs extending radially outward from the outer perimeter thereof. One or more bars affixed to the stator frame each includes a slot therein for engagement with the tabs extending from the lamellae. For present purposes, both of these techniques for aligning the lamellae in a stack for assembly of a stator correspond to each other and suffer the same drawbacks which the present invention seeks to overcome.
It is well known that conductor bars that are disposed parallel to the axis of a stator of a generator give rise to harmonic waveform degradation. One solution to this problem is to skew the slots in the interior of the stator annulus. This has been performed by skewing the dovetail-shaped key bars or slotted bars so that, when the lamellae are stacked thereon, the slots for the conductor bars are similarly skewed. A problem produced by such skewed guidance limits the amount of skew which can be tolerated.
The problem arises because the compression forces applied to the ends of the in-process stator are exerted parallel to the axis of the annulus. The guiding members (key bars or slotted bars) are skewed with respect to the axis of the stator. Up to about four degrees of skew, the differences between compression and guidance angles can be accommodated by manufacturing tolerances and deformation of material to attain satisfactory assembly. At skew angles exceeding about four degrees, the difference between the angle of applied compression force (axial) and the angle of lamellae guidance (skewed) results in the lamellae hanging up on the key bars or slotted bars. As a result of such hanging up, it is not possible to attain sufficient compression on the lamellae to produce a substantially rigid stator stack.